Note: Descriptions are shown in the official language in which they were submitted.
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STABILIZATION ~F ALUMINUM ELECTROLYTIC CAPACITOR FOIL
This invention relates to the stabilizatlon of
aluminum capaci.tor foil for use in an electrolytic capaci-
tor.
It has been well-documented that aluminum capaci-
tor foil after apparently complete formation of a highvoltage dielectric oxide film evidences instability as
shown by a sudden loss of field strength. This behavior
is most markedly observed when the foil also bears a hy-
drous oxide layer that was formed prior to anodization.
There is general agreement in the electrolytic capacitor
industry that this dielectric instability is caused by the
creation of voids within the formed dielectric oxide layer.
It has been further postulated tha-t oxygen gas is trapped
within these voids and is liberated during the treatment
("depolarization") that brings about a relaxation in the
strength of the dielectric.
Whatever the actual physical mechanism which
may be involved, it is known to remedy the situation by
various so-called depolarizing techniques -- heating,
immersion in hot water with and without various additives,
mechanical flexing, pulsed currents, current reversal,
or a combination of these -- in short, methods which tend
to relax or crack the dielectric barrier layer oxide so
that these voids may be filled with additional dielectric
oxide and thereby impart permanent stability to the oxide
film.
3~
When the anodization electroly-te is of the boric
acid or boric acid/borate type, the resulting oxide film
is attacked by water to form a non-insulating hydrous oxide.
When the anodization electrolyte is of the hydration-inhi-
biting type, e.g., citrate or phosphate, the film is notso readily attacked. This degradation can occur by water
in rinse baths, by the working electrolyte in the final
capacitor, and even by exposure to air, particularly with
borate dielectric films, not just by immersion in hot water.
Thus, one of the prior art stabilization techni-
ques, immersion in hot water, acts to open up the barrier
layer dielectric oxide and expose or heal the voids and
also forms hydrous oxide and/or attacks the dielectric
film. It is desirable to direct the process so that it
will relax or open the dielectric oxide film to permit
stabilization without seriously damaging the dielectric
film.
Various additives have been used in the prior
art in the hot water immersion stage to inhibit formation
of hydrous oxide while permitting stabilization or to
strip excess hydrous oxide from the foil. These additives
have proved beneficial, but more improvement is needed.
In accordance with this invention anodized alu-
minum foil is passed through a bath containing a borate
solution at a temperature of at least 80C and a pH of
8.5 to 9.5. After stabilization, the foil is reanodized.
Aluminum electrolytic capacitor foil is stabi-
lized after anodization by passage through a bath contain-
ing an aqueous borate solution at a temperature of at
least 80C and a pH of 8.5 to 9.5 and then reanodized.
preferably, the bath con-tains 0.001-0.05 moles/liter of
borax, the temperature is 90-100C, and at least two
stabilization cycles are used with three being preferable.
Borax or boric acid at acidic pH controls the
hydration of aluminum foil in aqueous solutions. At pH
8.5-9.5, the borax is more effective than the reaction
with water in opening up the dielectric film. The solu-
tion may form hydrous oxide in the voids which is converted
~ 3
-- 3
to barrier layer oxide in a subsequent re-anodization step.
At this pH, the borax may attack the excess hydrous oxide
present with minimum damage to the barrier layer dielectric
oxide, presumably because of the mildly alkaline conditions.
Whatever happens, a stable dielectric oxide is formed after
reanodization, i.e., the foil displays lower leakage cur-
rents and little change in capacitance. This stability is
tested by immersing the foil in boiling water and measuring
electrical properties in comparison to foil which has not
been so stabilized.
Anodized foil, preferably intermediate to high
voltage foil (200-750V) is stabilized by passing the foil
into a bath containing 0.001-0.05 moles/liter of borax at
a pH of 8.5 to 9.5 and a temperature of 90-100C. The
temperature is chosen to give adeq-uate reactlon time with-
out being unduly long. The residence time in the borax
bath should not be less than two minutes or there is incom-
plete reaction. More than eight minutes is unnecessary as
there is no further benefit in longer times. The actual
time is within this range and depends upon the speed at
which foil transport has been set. The temperature should
be at least 80C as below that temperature the reaction is
too slow to be useful. The pH is critical as at this pH
stabilization occurs without excessive damage to barrier
layer oxide. At least two stabilization-reanodization
cycles are used, with three preferred for best results.
Depolarization of high voltage foil is more
effectively carried out when the medium employed is a hot,
slightly alkaline aqueous solution. However, the pH can-
not be permitted to exceed 9.5 because of the rapid anduncontrollable dissolution of the aluminum oxide film that
may occur under such conditions. For that reason, it is
not feasible to use solutions of sodium hydroxide, potas-
sium hydroxide or the strong bases. Ammonium hydroxide,
a common and inexpensive base, is also not suitable because
of its great volatility and the attendant difficulty in
maintaining a solution of well-controlled composition.
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There are a large number of materials, namely
buffered solutions of weak acids, that will perform ade-
quately and whose pH can be easily controlled. Care must
be taken, however, to employ only those which will not be
lnjurious to the electrolytic capacitor constructed from
the foil. It is likely that even after extensive and
careful washing of the foil after complete processi.ng that
trace amounts of the compounds from the bath will persist
in the minute pores and recesses of the etched foil, and
it is naturally desirable that the presence of these
residues be innocuous in regard to capacitor properties
and reliability.
Among the possible acceptable substances that
lend themselves to this application are buffered solutions
of boric acid, phenol, phthalic acid, acetic acid, citric
acid, tartaric acid, and carbonic acid (mixtures of sodium
bicarbonate and sodium carbonate). However, borax is pre-
ferred as it is easy to control the composition and pH of
borax solutions, and it is non-injurious to aluminum foil.
Its anion, borate ion, also is commonly present in anodi-
zing electrolytes and capacitor fill or working electroly-
tes. Therefore, the use of borax reduces the possibility
of anion interference or competing reactions.
Example 1
In the following chart, seven oxide quality deter-
minations were made on high voltage (about 350V) foil, with
"1" being the highest rating. Knee volts and end volts
refer to the voltage from the aging curve where there is
an inflection following a rapid voltage change (knee point)
and final (end)voltage where the curve plateaus out.
Measurements were made both before and after boiling for
five minutes in deionized water. A "boil-build" test was
also performed in which the foil was immersed in boiling
water, re-anodized, and checked for instability, e.g.,
increase in capacitance and loss in dielectric strength.
Except for the "boil-build" test, the tests were run
using a glycol-borate electrolyte. Sample A was anodized
in a phosphate electrolyte, received no borax treatment,
5~3~7
but was boiled and reanodized. Sample BBB was similarly
anodized and was treated and re-anodized three times using
a 0.02-0.03 M borax treating solution at about 92C.
Table 1
Sample A BBB
End volts 5 5 2
Knee volts 5.5 2.5
Time to knee 4 4
End volts (boiled) 7 2
Knee volts (boiled) 6 2
Time to knee (boiled) 5
Boil-build 3
Total 36 14.5
Thus, it can be seen that the borax treatment increases
foil stability dramatically.
Example 2
It has also been shown that dielectric oxide
films prepared in boric acid electrolytes may be similarly
stabilized. In the table below laboratory results are
shown for 1, 2, and 3 treatments. The foil is initially
boiled to form the hydrous oxide layer and anodized to
400V, treated in the borax solution and reanodized, and
treated and reanodized twice more. Sample 1 was treated
with boiling water for 3 minutes while sample 2 was treat-
ed with a O.OlM borax solution for 30 seconds af-ter gas
evolution ("depolarization") occurs. There was an induc-
tion period with the borax solution and this was an easy
way to measure reaction time. Capacitance (Cap) is in
microfarads, dissipation factor (DF) as a percent, and
reform time in seconds, and capacitance and % DF were
measured after each reformation.
~ 3
-- 6 --
Table 2
Reform
Cap % DF Time Cap % DF
Sample 1
initial 4.127 6.6 -- -- --
1st treat 21.6 46.4 228 3.755 8.1
2nd treat 13.2 36.0 60 3.795 8.6
3rd treat 6.25015.2 15 4.134 8.2
Sample 2
initial 4.128 6.6 -- -- --
1st treat 10.5 17.8 74 4.208 6.5
2nd treat 9.7 18.2 29 4.217 6.2
3rd treat 6.88513.3 23 4.227 6.0
Capacitance increase îs a measure of the instabi-
lity of the dielectric oxide. It can be readily seen that
even after one borax treatment, the film is more stable
than with boiling water alone. More than three treatment
steps can be used, but any addltional improvement is slight
so as to make additional stages unjustifiable.
The process will work with less than O.OOlM borax
solution but below this the concentration is difficult to
control, and in very dilute solutions (pH of about 8 or
less) the solution begins to act as a hydration inhibitor
rather than showing the desired stabilization action. More
than 0.05 moles/liter of borax (2% solution) is not needed,and there are indications that more concentrated solutions
attack the dielectric oxide.
The specified pH range corresponds to the desir-
able borax concentrations and is a convenient process con-
trol. The temperature is maintained at 90C at least toprovide a desirable reaction rate. Preferably, it is kept
below 100C to keep the solution from boiling over and
to reduce violent bubbling as the foil enters the solution.
